A typical mechanical transmission for a vehicle or other machine permits only a selection between a few discrete gear ratios. A variable speed transmission, on the other hand, can be varied continuously within a finite range to provide a large or infinite number of possible gear ratios. This enables the relationship between the speed of the prime mover, such as a vehicle engine, and the driven speed of the rotary load, such as a wheel, to be selected within a continuous range. The result is a more efficient engine and improved fuel economy. The existing variable speed transmissions, however, can use improvements with respect to reliability, function, and simplicity.